Motor driven chain hoists find a wide range of application in industry and are particularly useful because of their versatility. Typical applications require a hoist assembly which is relatively compact and of reasonably light weight. In order to produce such a hoist, several factors need to be considered. For example, the chain itself must be as light in weight as is practical for handling the loads for which the hoist is designed; the electric motor should be light in weight and compact; and the hoist components must also be compact and light in weight. Moreover, the hoist must be capable of raising and lowering the load at a relatively rapid velocity.
To those skilled in the art, it will be obvious that the above general requirements impose certain interrelated physical restrictions on the hoist construction. For example, if the electric motor is to be compact and of light weight, it must be expected that its power output will be relatively low. Consequently, the drive motor must operate to drive the hoist chain with high mechanical advantage if the load capacity of the hoist and the raising and lowering speeds are to be practical. This dictates the use of a drive sprocket which is of small diameter and which rotates at a relatively high angular velocity.
However, the smaller the drive sprocket, the more pronounced is the unevenness of motion inherently imparted to the chain as the load is raised or lowered. That is to say, since the chain is a flexible element comprising articulated chain links of finite lengths, the vertical motion of the chain in raising or lowering a load is the resultant of two motions. First, there is the constant velocity vertical motion which is a function of the angular speed of the sprocket wheel and its effective diameter and, superimposed upon this constant velocity vertical motion, there is an oscillatory vertical motion which is a function of the fact that the chain does not smoothly train over the sprocket wheel. Stated otherwise, it is impossible to impart a completely smooth drive motion to a chain when the drive is effected by a rotating drive sprocket. Moreover, the amplitude of the oscillatory motion increases as the size of the drive sprocket is decreased.
This superimposed oscillatory motion imposes additional alternating stresses upon the entire system over and above those induced by the steady load and since the amplitude of such oscillatory motion is increased as the effective diameter of the sprocket wheel is reduced, the added stress conditions are exacerbated by the above-noted requirements of a powered chain hoist.
Moreover, an additional problem arises because of the superimposed oscillatory motion described. This problem has to do with the fact that the load system which comprises the hoist, its support and the load which is suspended will possess a natural frequency due to spring rate characteristics inherent in the system and dependent upon the mass of the load. Since the inherent resiliency of the chain affects the spring rate of the system and since the effective length of that portion of the chain which supports the load is continuously changing as the load is raised or lowered, the natural frequency of the system likewise is constantly changing during raising or lowering. Consequently, if the oscillatory excitation due to the drive sprocket creates resonant response in the load system as the active length of the chain between the hoist and load approaches some value within the operating range of the hoist, the amplitude of the aforesaid oscillatory motion can become quite large and correspondingly large, increased stresses may be imposed upon the system.
Thus, even though the increased stresses do not exceed the ultimate strength of the chain or components of the hoist, they can become large enough to exceed the endurance limit of the chain and/or hoist components.
That is to say, the hoist and chain normally will be designed such that, absent the resonant conditions specified above, the endurance limits of the chain and hoist components are not exceeded, usually even in the presence of significant overload such as 150% of rated load. Theoretically, then, the hoist assembly would not fail in fatigue since it would be able to withstand an infinite number of stress cycles. However, the increased stresses produced by the aforesaid resonant conditions may well be sufficient to exceed the endurance limits of the chain and/or hoist components, causing fatigue failure after some finite number of stress cycles.
To overcome the above problem, attempts have been made to design systems in which the active length of the chain at which resonant response occurs lies outside the operating range of the hoist, or a limiting active length has been specified, outside of which the hoist should not be operated. Alternatively, the hoist operating speed may be reduced to avoid resonance. These solutions are not entirely successful because each limits the versatility of the hoist.
Another approach has been to introduce a resilient device in series with the chain and load to lower the range of natural frequencies which the system exhibits during raising and lowering, the lower natural frequency being such as to move any resonant condition outside the operating range of the hoist. This solution can be acceptable under limited circumstances and has been effected by the use of a stack of Belleville washers incorporated in the load hook of the hoist. Specifically, the Belleville washers introduced a soft enough spring element into the system as to require such a short active chain length at which resonance occurs as to be outside the operating range of the hoist. An undesirable side effect of this approach is that the soft or weak load hook spring allows considerable bouncing of the load during raising and lowering as well as a persistent transient oscillation induced by stopping the load. This is undesirable for several reasons. First of all, it makes it difficult accurately to position the load with the hoist. Further, noticeable bouncing of the load imparts the appearance of an unsafe condition even though none actually exists and an operator or persons standing nearby the load are understandably uncomfortable. Also, it can happen that the load is delicate or fragile and cannot tolerate the bouncing incurred.